The present invention relates generally to junction field effect transistors and, more specifically, to an improved thin channel junction field effect transistor.
Junction field effect transistors (JFET) have been used as active devices for many years. More recently, a JFET structure which is compatible with bipolar processing known as a BIFET has been developed. Parallel source and drain regions are formed in a bottom gate region during the base diffusion of the bipolar processing. A common ohmic contact region to the bottom gate region and the to-be-formed top gate region are formed during the emitter diffusion of the bipolar devices. The additional BIFET processing steps are ion implantations of a channel region between the source and drain regions and buried below the surface followed by an ion implantation of the top gate region. Since in most applications, the top and bottom gates are connected together, a single ohmic contact to the bottom and the thin top gate regions is used. Also, the top gate extends outside the channel region and makes direct contact with the bottom gate. Such a prior art device is illustrated in FIG. 1.
The P implant which forms the channel region is about 0.3 microns thick and about 0.3 microns below silicon surface. The two P diffusion contacts which form the source and drain along the two edges provide a path from the channel implant up to the top surface wherein interconnects can be made. The N implant forms the top gate and is very shallow typically about 0.3 microns into the top surface. The N top gate implant is lower in impurity concentration than the surface of the P source and drain diffusion and, consequently, aluminum cannot make an ohmic contact thereto. Also, the top gate is so shallow that aluminum will migrate through it to form a short to the channel region if direct contact were made. To avoid these problems, the gate implant is extended and overlaps the N+ contact diffusion at the end of the channel.
In many applications, it is desirable to isolate the top gate from the bottom gate so they can be connected to separate terminals. This is particularly desirable when gate leakage or AC performance is important. In both cases, the presence of isolation region parasitics on the bottom gate make it desirable to disconnect the bottom gate from the input from the signal input or top gate. Another advantage afforded by an isolated gate structure is that several devices can be built in a common bottom gate isolated island rather than in separate isolated islands. This saves die area and improves match of matched pairs by allowing the members of the pair to be closer to one another.
The basic method used in the prior art to isolate top and bottom gates is to form the top gate as a closed geometry surrounding either the source or drain and the top gate surrounded by the other terminal. The choice of a closed geometry provides termination of gate region across the source and drain contact regions along the entire parameter of the gate. This assures that there is no contact between top and bottom gates along their edge. An example of such a closed geometry JFET is illustrated in U.S. Pat. No. 3,649,385.
Thus, there exists the need for a thin channel junction field effect transistor which has an isolated top and bottom gate.